1. Field of the Invention
The present invention relates to an imaging optical unit and an inspection method for the same. Particularly in an image scanner, a digital copying machine, a facsimile, or the like, the present invention can be used suitably for reading image information of a monochrome image or a color image using a signal from a line sensor (image pickup element or photoelectric conversion element).
2. Description of the Related Art
Conventionally, various flat bed type image scanners have been proposed as an image reading apparatus (image scanner) for reading image information on a surface of an original (see Japanese Patent Application Laid-Open No. H03-113961).
In the flat bed type image scanner, an imaging lens (imaging optical system) and a line sensor (CCD) are fixed while only a reflection mirror disposed in an optical path is moved so that the surface of the original is scanned by slit exposure for reading image information, which is called a 2:1 mirror scanning method.
In recent years, in order to simplify a structure of the apparatus, the reflection mirror, the imaging lens, the line sensor, and the like are incorporated as one unit to scan the surface of the original. This is called an integrated carriage type scanning method and has been adopted in many cases.
FIG. 9 is a schematic diagram of a main part of a conventional image reading apparatus adopting the integrated carriage type scanning method.
In FIG. 9, a light beam emitted from an illumination light source 81 illuminates an original 87 placed on an original table (original table glass) 82. Further, a reflection light beam from the illuminated original 87 bends its optical path inside a carriage 86 while being reflected by first, second and third reflection mirrors 83a, 83b and 83c in turn, and an imaging lens (imaging optical system) 84 condenses the light beam on a surface of a line sensor (image pickup element) 85 so as to form an image.
The line sensor 85 includes multiple light receiving elements aligned in a one-dimensional direction. In a case of FIG. 9, the light receiving elements are aligned in a direction perpendicular to a paper surface (corresponding to a main scanning direction).
Further, the carriage 86 is moved by a driving motor 88 in a direction of an arrow A illustrated in FIG. 9 (corresponding to a sub scanning direction) so that image information of the original 87 can be read in a two-dimensional manner.
FIG. 10 is an explanatory diagram of a fundamental structure of an imaging optical system of the image reading apparatus illustrated in FIG. 9.
In FIG. 10, numeral 84 denotes an imaging lens. A group of line sensors 85 includes line sensors 85R, 85G and 85B for reading red (R) color, green (G) color and blue (B) color. Read areas 87R, 87G and 87B on the surface of the original 87 correspond to the line sensors 85R, 85G and 85B, respectively. The surface of the original 87 is scanned in the sub scanning direction indicated by the arrow, whereby the same area is read by the line sensors 85R, 85G and 85B of different colors with a certain time interval.
Conventionally, it has been proposed variously that a non-coaxial optical system with aberration being corrected can be constructed by introducing a concept of a reference axis instead of an optical axis as the imaging optical system that is used for an image formation apparatus, in which a constitution surface becomes an asymmetric aspherical surface (see Japanese Patent Application Laid-Open No. H08-292371).
Japanese Patent Application Laid-Open No. H08-292371 discloses its designing method.
In addition, as an imaging optical system having a simpler structure, an imaging optical system having a small number of free-form curved surface mirrors (non-coaxial optical system) has been variously proposed (see Japanese Patent Application Laid-Open No. 2004-133378).
An off-axial optical system using optical elements having a free-form curved surface mirror has a non-coaxial constitution surface. With no vignetting occurring on a reflection surface, there is a merit that an optical system using the reflection surface can be constructed easily.
In addition, the off-axial optical system having only the reflection surface has a merit that no chromatic aberration occurs. Therefore, it is possible to meet a high demand about a numeric aperture or resolution because there is no color blur or shift unlike a conventional refraction optical system.
However, the off-axial optical system including the optical element having the reflection surface has a tendency to decrease its optical performance largely if a shape of the optical element changes or an off-center error occurs in the manufacturing process of the optical element.
Therefore, a reflection type optical element is required to have higher accuracy of a surface shape of each optical element and assembly thereof when assembled in a case compared with the case of using an optical element of a refraction system.
When the optical elements are housed and held in the case, it is very difficult to measure changes of shapes of the optical elements and an assembly error in the state where the optical elements are held, because the optical path is bent by the reflection and the reflection surfaces face each other inward in the entire system.
In particular, if design optical performance cannot be obtained as a whole, it is very difficult to specify which optical element is the cause.
Conventionally, all the optical elements should be removed from the case for measuring the change in shape (surface shape) of each optical element, and then each optical element is reassembled in the case if it has no problem.
Therefore, as to the imaging optical unit including the reflection type optical elements having the free-form curved surface, it is very difficult to inspect the change in shape of each optical element and to measure the assembly error or the like after the optical elements are once housed and held in the case.
In order to reduce a relative positional error of the optical elements and to increase the assembly accuracy, it is necessary to manufacture a holder member for holding the optical element with high accuracy. However, it is very difficult to manufacture the holder member with high accuracy.